During the course of gene transfer experiments into K562 cells using a modified adenovirus vector, it was noted that control K562 cell DNA showed a positive signal when probed with the entire adenovirus type 5 genome. Subsequently we have determimed that this signal is also present in all human genomic DNA tested. An Sst I digest of human genomic DNA probed with the entire adenovirus 5 disclosed 3 bands that were shown to be homologous to distinct portions of the adenovirus genome. A sequence was isolated from a human cosmid library that corresponded to a particularly strong signal at 2.5 KB on a genomic human Sst I digest, probed with the fist 9 Kb of the 5' end of adenovirus 2. This isolated clone has been shown to contain a 2.5 Kb Sst I frament homologous to a portion of the E1B coding region of adenovirus 2 and 5. It also has its own interesting structure in that this unit is repeated in tandem fashion approximately ten times in the cosmid clone. We have tenatively mapped this sequence, through the use of somatic cells, to human chromosome 4. This copy number of this sequence has been estimated as greater than 300. At present in situ hybridization is being performed and we are beginning to sequence the 2.5 Kb Sst I fragment.